Acute Disseminated Encephalomyelitis

Acute disseminated encephalomyelitis (ADEM) is an uncommon demyelinating disease of the central nervous system. It is an immune-mediated inflammatory process that may occur after a viral infection or vaccination, in association with rheumatic fever, or without a recognized trigger.

CT and MR findings are nonspecific and the differential should include infectious and other demyelinating conditions. There are multiple lesions in the white matter and basal ganglia with or without spinal cord involvement. The gray matter may also be involved.

MRI will show bilateral, asymmetric foci of increased signal on T2 and FLAIR sequences, predominantly involving the gray-white interface and sparing of the periventricular white matter (in contrast to multiple sclerosis). The lesions demonstrate variable restricted diffusion in acute lesions. ADC maps will reveal increased signal. However, unlike multiple sclerosis, the normal-appearing white matter will demonstrate normal ADC values. The lesions may have punctate, ring/incomplete ring, or peripheral enhancement. There may also be enhancement of the cranial nerves.

Differential considerations, as alluded to above, include:

• Infection: Ring enhancement may be similar.
• Multiple sclerosis: May have identical imaging features, so repeat MR is needed to differentiate ADEM (monophasic) from multiple scelrosis. Multiple scelrosis does have a tendency to involve the periventricular white matter and has lesions that are often more symmetric in distribution.
• Autoimmune-mediated vasculitis
• Fabry disease: Scattered non-enhancing white matter lesions in patients with renal failure or heart disease.
• Behçet: Tendency to involve the midbrain. Lesions show nodular enhancement in acute phase.

References

• Mader I, Stock KW, Ettlin T, Probst A. Acute disseminated encephalomyelitis: MR and CT features. AJNR Am J Neuroradiol. 1996 Jan;17(1):104-9.
• Singh S, Alexander M, Korah IP. Acute disseminated encephalomyelitis: MR imaging features. AJR Am J Roentgenol. 1999 Oct;173(4):1101-7.